The invention concerns a method for coupling a gas chromatograph to an NMR spectrometer, wherein the carrier gas present at the outlet of a separating column of the gas chromatograph, including a sample contained in the carrier gas, is supplied via a heated transfer line to a collecting device for the sample contained in the carrier gas.
A method of this type is e.g. used with the “preparative fraction collector PFC” (Präparativer Fraktioniersammler PFC) of the company Gerstel GmbH & Co. KG, Mülheim an der Ruhr (see their manual, issued December 1999).
Nuclear magnetic resonance (NMR) spectroscopy is an important method for the analysis of chemical substances. The behavior of the nuclear spins of the substance to be analyzed is thereby measured during excitation in a strong magnetic field.
The test samples to be investigated are often a mixture of the most different of chemical substances. When all components of the mixture are measured at the same time, the resonance lines of the substances overlap. This complicates determination of the individual chemical substances.
In order to simplify determination of the individual chemical substances, the test sample to be investigated is conventionally separated prior to the NMR measurement. One of these separating methods is gas chromatography (GC). GC is used, in particular, for separating test samples that contain non-polar substances. The test sample is introduced into a separating column through which a carrier gas (e.g. nitrogen gas) flows. The separating column is then slowly heated, and the individual substances of the test sample gradually pass over into the gaseous phase and are transported away by the carrier gas. Below, the substance(s) contained in the carrier gas is/are referred to as the sample. The sample is conventionally initially collected, subsequently dissolved in a solvent, and measured in the NMR spectrometer.
A heated transfer line is connected to the separating column of the PFC apparatus of the company Gerstel and is guided to six collecting tubes via a distributor valve. The collecting tubes are designed as open U-shaped tubes which are cooled in their lower region. The sample condenses out in this lower region. When a sufficient amount of substance has condensed (which usually requires multiple feeding of the separating column and heating cycles), the sample is manually washed out of the collecting tube with a solvent, concentrated, transferred to an NMR sample tube, and is subsequently measured.
In this conventional method, a relatively large amount of sample is lost between the separating column and the NMR spectrometer, initially through the open collecting tube. The sample is not completely condensed on the wall of the collecting tube, in particular, when the collecting tubes are short, but part thereof exits through the open end. Sample substance is also lost when the sample is washed out of the collecting tube and filled into an NMR sample tube. If only little test sample is available, i.e. the loss of sample cannot be compensated for through accumulation over several GC runs, sample loss deteriorates the signal-to-noise ratio of the NMR measurement, or even renders NMR measurement impossible.
Since both the distributor valve and the transfer line must be heated in order to prevent condensation of the sample before it reaches the collecting tube, or even clogging of the line, the number of collecting tubes is also usually limited (in the system “PFC”, limited to six collecting tubes).
It is the underlying purpose of the present invention to provide a method for coupling a gas chromatograph to an NMR spectrometer with which the sample loss is reduced. Fundamentally, a method of this type should also considerably increase the number of possible collecting locations and reduce manual interactions.